1. Field of the Invention
The present invention relates generally to a Gauss type lens system and more particularly to a modified Gauss type lens system in which the doublet in the second lens group of the typical 4-group, 6-element Gauss type lens system is separated into two single lens elements to form a 5-group, 6-element lens system.
2. Description of the Prior Art
As is known in the prior art, an exchangeable objective lens for use in a single lens reflex camera requires a sufficient back focal distance to allow the movement of the swingable reflex mirror in the camera body. A Gauss type or modified Gauss type lens system, however, is generally difficult to be designed to have a relatively great back focal distance with various aberrations well corrected. In fact, the back focal distances of conventional Gauss type or modified Gauss type lens systems are only about 70 percent of the focal length of the system, if they are designed to be of aperture ratio, about F/2.0 or still higher aperture ratio.